The present invention relates to channel estimation in a mobile receiver when transmit diversity is used. In particular, the present invention is related to channel estimation in a transmit diversity environment using pilot signals.
Transmit diversity is a technique involving transmitting data from two antennas and is used to improve fading margins. More specifically, in radiocommunication systems employing transmit diversity and corresponding diversity reception, a signal is obtained by combining signals that originate from two or more independent sources that have been modulated with identical information-bearing signals that may vary in their transmission characteristics at any given instant. Diversity transmission and reception are used to obtain reliability and signal improvement by overcoming the effects of fading, outages, and circuit failures. In radiocommunication systems using diversity transmission and reception, the amount of received signal improvement depends on the independence of the fading characteristics of the signal as well as circuit outages and failures.
Fading in a radiocommunication system may refer to the time variation of the amplitude or relative phase, or both, of one or more of frequency components of a received signal. Fading may be caused by time varying changes in the propagation path characteristics of the medium typically an air interface in the case of radiocommunication systems. Accordingly, a fading margin, may be defined as a design allowance providing for sufficient system gain or sensitivity to accommodate expected signal fading, for the purpose of ensuring that a required quality of service is maintained throughout, for example, a geographic region or service area. A fading margin may express the amount by which a received signal level may be reduced without causing system performance to fall below a specified threshold value which would compromise reception by, for example, a mobile station (MS).
Transmit diversity may be achieved, for example, with independent transmissions from two antennas. By transmitting from two antennas, associated with, for example, two Base Stations (BS), the integrity of the communication is less sensitive to disruption should signal fading occur on one or both of the propagation paths due to channel noise or other changes to the propagation path characteristics. A BS may transmit both data and control signals to an MS. It may further be common for predetermined signals, for example, pilot signals, to be transmitted from the BS to the MS.
A pilot signal is a signal, usually transmitted at a single frequency over, for example, the air interface of a radiocommunication system for supervisory, control, equalization, continuity, synchronization, or reference purposes. Because of the proliferation of frequency channel hopping systems, it may sometimes be necessary to employ several independent pilot frequencies to convey pilot related information. Pilot signals, in such systems, may include a series of predetermined symbols. Such predetermined symbols may provide known information which can be used by a receiver to perform channel estimation.
In the case of systems employing transmit diversity, different pilot signals, or pilot symbols, are transmitted on the two antennas. This makes it possible for the MS to derive individual channel estimates for signals transmitted by the two BS antennas. Channel estimates allow the receiver to characterize the manner in which the air interface is impacting the transmitted symbols by generating, for example, intersymbol interference (ISI). Accordingly, a channel estimate facilitates the recovery of information from the received signal in the event of signal fading and the like. Channel estimates using known pilot signals provide the receiver with some information about the nature of the received signal and is an example of coherent estimation while still other systems attempt to characterize the air interface without any a priori knowledge of the received signal.
To provide an opportunity for an MS to make individual channel estimates based on signals transmitted from two antennas, as in a transmit diversity scenario, different pilot symbol sequences are transmitted on the two antennas. One such pilot pattern scheme is described in the 3GPP-standard, see 3rd Generation Partnership Project (3GPP), 3GPP RAN 25.211 V2.1.0, June 1999, Section 5.3.2, the disclosure of which is incorporated herein by reference.
Channel estimation methods using transmit diversity encoded pilots have relied on grouping, for example, alternate pilot symbols to generate two sets of channel estimates. Prior art systems make the assumption that the channels over which the pilot symbols are transmitted are constant for a duration of, for example, three or more symbols. A problem arises however in that the assumption that the channel is constant for three symbols may be inaccurate. Such an assumption may be particularly problematic in a transmit diversity scenario with rapidly time-varying channels, for example, resulting from a high degree of Doppler shift coupled with, for example, a high spreading factor.
Problems may also arise generally in connection with a high spreading factor. The spreading factor associated with, for example, a DS-CDMA system refers to the length of a symbol in chips, e.g., the number of transmitted bits or symbols in a spreading sequence per information bit or symbol. A high spreading factor refers to a symbol with a long duration. It can be appreciated that symbols of a longer duration may be more susceptible to a variety of anomalies caused by time varying channel characteristics.
It would therefore be appreciated that a need exists in the art for a method and apparatus for improving channel estimation in a transmit diversity system which does not rely on a number of consecutive pilot symbols being constant in order to form a channel estimate. Such a method and apparatus would lead to reduced channel overhead and improved bandwidth for information bearing payloads and improve symbol detection.